The relations among (1) temporary loss of auditory sensitivity, or temporary threshold shift (TTS), (2) permanent threshold shifts (PTS), and (3) damage to cochlear structures caused by exposure to noise will be studied in the chinchilla, with the ultimate goal the prediction of susceptibility of a given ear to PTS and/or to structural damage on the basis either or TTS data or of other measurements that can be made on the intact organism. Studies similar to those already conducted on the sequelae of a relatively broad-band noise will be performed using narrow-band noises. Considerable effort will be devoted to study of the equal-energy hypothesis (which postulates that PTS and cochlear damage depend only on the total relevant acoustic energy absorbed by the ear, regardless of temporal pattern of the noise), because of the practical significance of this principle in regard to damage-risk criteria. The physiological processes associated with recovery from severe noise exposures will also be studied by electron microscopy, particularly the development, shortly after exposure, of small osmiophilic particles and larger inclusion bodies (thought to be deposits of glycogen) in the stria vascularis. Possible changes in permeability of cochlear partitions immediately after intense noise will be studied by means of the tracer horseradish peroxidase. The influence on TTS and PTS of certain medications said to ameliorate the effects of acoustic trauma will also be investigated.